It is known to treat a bed of bulk material with gas by conveying it continuously over a grate so that the gas flows through it. To cool combustion material, for example cement clinker, it is customary to use what are known as pusher grates, which comprise overlapping rows of, in alternation, stationary grate plates and grate plates which are moved forward and back in a conveying direction (DE-A 3 734 043). The grate plates are used to blow cooling air into the bed of material, and this air is discharged above the bed of material in order for the heat to be recovered. For economic operation, pusher grates require complex mounting of the moving parts and are also complex since they are composed of relatively small parts. Another known design of grate makes use of a stationary, air-permeable supporting base, over which the layer of material is moved by means of scrapers which are moved continuously in the conveying direction or by means of reciprocating pusher members (EP-A 718 578; DE-A 10018142). The scrapers or pusher members have to be guided moveably through the grate surface from below, which is a complex arrangement. Moreover, they are exposed to high levels of wear within the hot layer of material. The passage of air is impeded and the cooling action restricted in the region of the grate in which they and their driving and sealing members are located. Yet a further type of cooler (DE-A 101 13 516) uses a large-area grate which is moved forward and back in its entirety, the bed of material being held in place by a blocker plate arranged at the start of the grate during the return stroke, so that the grate can slide beneath the blocker plate. This has the drawback that the grate length which can be utilized is limited on account of the barrier formed by the blocking action.
Working on the basis of the above prior art, the invention is based on the object of providing a method and an apparatus for treating, in particular cooling, a layer of bulk material which rests on a grate and has a stream of gas passing through it, which promise a simple structure and simple operation as well as uniform treatment of material even with a considerable grate length.
The method according to the invention uses a grate which comprises a plurality of planks which are elongate in the conveying direction. A plurality of these planks are moved forward and back in the conveying direction in order to move the material in the conveying direction. The movement is controlled in such a way that each individual plank in each case moves forward in the conveying direction together with at least one adjacent plank and in each case moves back at a different time from the adjacent planks. In this case, the width of the planks is made so narrow that during the backward movement of a plank the backward conveying action exerted on the material above it, which is based on the friction between the plank in question and the material, is lower than the forward conveying action or holding action which is based on the friction exerted by the respectively adjacent material or by the side walls. The result of this is that when a plank moves back between two stationary planks, the strip of material above it is mostly or completely held in place by the material located above the adjacent planks, and consequently to this extent does not follow the plank which has been moved back individually. When two or more adjacent planks together with the material located thereon move simultaneously in the conveying direction, surrounding an individual plank which is stationary or moving in the opposite direction, they also carry with them the or at least a significant part of the strip of material located on the latter plank. By suitable movement control, in which the return stroke of each plank is controlled in such a way that the adjacent planks hold the material located thereon in place while the forward stroke is always carried out by a plurality of adjacent planks simultaneously, the bed of material is conveyed in the conveying direction.
This conveying principle has long been known (DE-B 1 296 087, U.S. Pat. Nos. 3,534,875, 4,144,963). In the field of conveying technology, this principle does not play a major role, since it is generally simpler for the layer of material to be moved by means of pusher members or scrapers which move above the supporting surface. By contrast, in the context of the invention it constitutes a particular advantage, since there are no conveying members above the grate which would be exposed there to the possibly aggressive action of the treatment atmosphere or the material.
A further advantage of the invention consists in the fact that the mixing action exerted on the bed of material by the conveying movement is low. In particular, there is a lack of vertical mixing movement in the bed of material. The result of this is that in the case of a cooling process the treatment gas passes through the hottest layers last and the result is an improved recovery of heat compared to what is possible with more lively vertical mixing of the bed. Corresponding statements can also be made with regard to other types of heat or mass transfer. However, if a more intensive movement of material is desired, for example in order to avoid caking, this can be brought about by the forward conveying not taking place uniformly over the entire width of the grate, but rather in sections, so that a relative movement is generated between these sections. Another possible way of moving the material to a greater extent consists in selecting a higher grate frequency. Finally, a movement within the bed of material can be produced by stationary internals which project into the bed.
The return stroke is expediently faster than the forward stroke. Furthermore, it may be expedient if the plurality of planks constantly moves at the same speed in the conveying direction. This makes it possible to ensure that the layer of material always moves at a substantially constant speed without energy losses or disruptive shaking being produced by coming to a standstill and starting up again.
In the case of a combustion material cooler, it may be expedient for at least one plank close to the edge to be moved at a lower frequency and/or amplitude than the planks located further toward the inside, in order in this way to prevent the material at the edge of the bed from flowing undesirably quickly. Instead or in addition, it is possible to provide stationary edge planks. It is also possible for the other moving planks to be controlled differently with regard to frequency and amplitude, so that it is possible to take account of different demands imposed on material over the width of the grate. Specifically, depending on the nature of the combustion furnace connected upstream of the cooler and/or of the furnace discharge, it is possible that the properties of the material will not be uniform over the width of the bed. In order nevertheless to achieve a conveying speed which is identical or, depending on circumstances, deliberately different, it may therefore be desirable for the conveying movements of the planks in one part of the grate to be more intensive than in another part of the grate. A different conveying action on the part of the planks can also be achieved by some of them from time to time not participating in the stroke of the planks which are otherwise moved with them. By way of example, it is possible to provide for a plank to participate only in every other conveying stroke of the adjacent plank or for it to miss out on every seventh conveying stroke.
Finally, it is also possible to arrange suitable brakes which are formed by stationary obstacles in regions in which experience has shown that the material tends to reach higher speeds or even to shoot straight through. By way of example, stationary intermediate planks, which if appropriate may also be provided with flow obstacles projecting into the bed of material, may be provided between adjacent planks. It is also possible for internal fittings which project over the grate from the side walls to be provided, so that the uniform structure of the base does not have to be disrupted by internal fittings. The internal fittings which project over the grate from the side could have a height which is lower than the normal bed height, so that they constantly engage in the bed in order to prevent its edge regions from flowing more quickly. If it is merely necessary to prevent material above the normal bed from shooting straight through, their height is selected in such a way that they are located where such a phenomenon is to be expected, i.e. above the normal bed height.
The gap between adjacent planks is expediently sealed in order to prevent material from dropping through the grate and/or to prevent gas from passing through. The seal used is preferably a labyrinth seal in which a longitudinal strip which is secured to the edge of the plank is immersed in a bed of the material which is held by the adjacent plank or a special channel. A simple design of this principle is known (U.S. Pat. No. 5,222,593), in which an edge strip projects upward from the edge of one plank and an edge strip which projects downward from the other plank engages over it. In this case, however, the seal is located in the aggressive region of the material which is to be treated. Therefore, according to the invention preference is given to an embodiment in which the sealing arrangement is arranged beneath the planks. In this case, a longitudinal channel which is open at the top and in operation receives a bed of the material is arranged in a stationary position at the edge of one plank or separately. A longitudinal strip projects from the edge of the plank into this channel and into the bed which forms therein. Together, these parts form a seal preventing bulk material which enters the channel through the gap located between the planks from passing through. Moreover, the arrangement forms a seal or at least one flow obstacle to gas between the space beneath the grate and the space above the grate. A limited passage of gas may be desirable, so that the desired gas treatment in the bed of material can also take place above this sealing device. For this purpose, it will in many cases be sufficient for one side of the upwardly open channel to be connected to the space beneath the grate.
According to a particular feature of the invention, the sealing device or a stationary intermediate plank located between two adjacent planks is supported by a stationary wall which separates the spaces beneath the grate below the adjacent planks from one another. This enables the material located on adjacent planks to be ventilated to different extents.
During operation, the strip which projects downward into the channel from the edge of a plank should always maintain a sufficient spacing from the channel walls, so that the material is not crushed between the strip and a channel wall. This can be achieved by making the lateral spacing between the strip and the channel walls greater than the lateral guide clearance of the planks including any thermal expansion. Furthermore, for the same purpose it may be expedient for the gap between the bottom edge of the strip and the base of the channel to be smaller than the gap between the strip and the channel wall.
If the channel is provided in a stationary position, a longitudinal strip may project downward into this channel from each of the two plank edges which are to be sealed with respect to one another. However, it is also possible for the channel to be connected to the edge of one plank and for the downwardly projecting strip to be connected to the edge of the other plank.
That part of the channel which is in direct communication with the plank gap will generally be completely filled with bulk material. During the relative movement of the strip and the channel with respect to one another, some bulk material will also pass through to the other side beneath the strip. The vertical spacing between the lower edge of the strip and the channel base should be small, so that this passage of material is inhibited. Moreover, the channel walls should be high enough to reliably prevent the material from overflowing into the space beneath the grate. An overflow of this type is unlikely because the channel is constantly emptying itself. Because the strips of adjacent planks are moved forward together and back individually, the bed of material located in the channel will be moved to a greater extent in the conveying direction than back and will be pushed out of the end of the channel, which should be open for this purpose. Since there is no risk of the material moving backward, the rear end of the channel, adjacent to the feed end of the grate, can be open.
It is customary to connect a short section of grate whose feed surface is particularly suitable for direct feed of the material which drops out of the furnace and which is consequently referred to as a feed section upstream of the grate of a combustion material cooler. During their movement, the feed-side ends of the planks of the grate section according to the invention move more or less far under the feed section. For the relative movement, they require a certain clearance between their top side and the underside of the feed section. To prevent cool air from entering the bed of material from the space beneath the grate, which is under excess pressure, as a result of this clearance without having flowed through and cooled the grate, it is expedient to provide a sealing arrangement in the region of which this clearance is reduced.
Advantageously, the top side of the grate is provided over substantially its entire surface with hollows in which cooled material collects, preventing direct contact between the grate and hot layers of material. The feed-side ends of the planks are expediently also equipped with hollows of this type. When they move under the feed section of the cooler and there is no longer any load from the bed of material at that location, there is a risk of the treatment gas which penetrates into the hollows from below blowing out the contents of the hollows or at least the fine material. According to a further feature of the invention, therefore, there is provision for the air passage openings in the hollows to be blocked off from the supply of air when they are located beneath the feed section of the cooler.
An important advantage of the invention consists in the fact that the wear to the grate and its bearing members does not impair its function. Therefore, the bearing members can be of simple configuration. By way of example, the grate may be mounted on rollers.